Radiation image recording and reproducing method

ABSTRACT

A divalent europium activated alkaline earth metal halide phosphor having the formula (I): 
     
         M.sup.II X.sub.2 ·aM.sup.II X&#39;.sub.2 :xEu.sup.2+  (I) 
    
     in which M II  is at least one alkaline earth metal selected from the group consisting of Ba, Sr and Ca; each of X and X&#39; is at least one halogen selected from the group consisting of Cl, Br and I, and X≠X&#39;; and a and x are numbers satisfying the conditions of 0.1≦a≦10.0 and 0&lt;x≦0.2, respectively. A process for the preparation of said phosphor, a radiation image recording and reproducing method utilizing said phosphor, and a radiation image storage panel using said phosphor are also disclosed.

This is a divisional application of Ser. No. 07/336,553 filed 4/11/89,which is a continuation of Ser. No. 07/219,848, filed 7/11/88, which wasa continuation of Ser. No. 06/834,886, filed 2/28/86, which was acontinuation of Ser. No. 06/660,987, filed 10/15/84, all now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a novel phosphor, a process for thepreparation of the same, a radiation image recording and reproducingmethod utilizing the same, and a radiation image storage panel employingthe same. More particularly, the invention relates to a novel divalenteuropium activated alkaline earth metal halide phosphor.

2. Description of the Prior Art

There is well known a divalent europium activated alkaline earth metalfluorohalide phosphor (M^(II) FX:Eu²⁺, in which M^(II) is at least onealkaline earth metal selected from the group consisting of Ba, Sr andCa; and X is a halogen other than fluorine), as a divalent europiumactivated alkaline earth metal halide phosphor. For instance, JapanesePatent Publication No. 51(1976)-28591 discloses that the phosphor givesan emission (spontaneous emission) in the near ultraviolet region whenexposed to a radiation such as X-rays, cathode rays or ultraviolet rays,the maximum of the emission being at the wavelength of approx. 390 nm,and the phosphor is useful for a radiographic intensifying screenemployable for radiography such as X-ray photography.

Recently, it has been discovered that the divalent europium activatedalkaline earth metal fluorohalide phosphor emits light in the nearultraviolet region when excited with an electromagnetic wave such asvisible light or infrared rays after exposure to a radiation such asX-rays, cathode rays and ultraviolet rays, that is, the phosphor givesstimulated emission, as disclosed in Japanese Patent ProvisionalPublication No. 55(1980)-12143. For this reason, the phosphor has beenpaid much attention as a phosphor for a radiation image storage panelemployable in a radiation image recording and reproducing methodutilizing a stimulable phosphor.

The radiation image recording and reproducing method can be employed inplace of the conventional radiography utilizing a combination of aradiographic film having an emulsion layer containing a photosensitivesilver salt and an intensifying screen as described, for instance, inU.S. Pat. No. 4,239,968. The method involves steps of causing astimulable phosphor to absorb a radiation having passed through anobject or having radiated from an object; sequentially exciting (orscanning) the phosphor with an electromagnetic wave such as visiblelight or infrared rays (stimulating rays) to release the radiationenergy stored in the phosphor as light emission (stimulated emission);photoelectrically detecting the emitted light to obtain electricsignals; and reproducing the radiation image of the object as a visibleimage from the electric signals.

In the radiation image recording and reproducing method, a radiationimage is obtainable with a sufficient amount of information by applyinga radiation to the object at a considerably smaller dose, as comparedwith the conventional radiography. Accordingly, the radiation imagerecording and reproducing method is of great value, especially when themethod is used for medical diagnosis.

As for a stimulable phosphor employable in the radiation image recordingand reproducing method, a rare earth element activated alkaline earthmetal fluorohalide phosphor such as the above-mentioned divalenteuropium activated alkaline earth metal fluorohalide phosphor is known,but almost no stimulable phosphor other than this phosphor is known.

SUMMARY OF THE INVENTION

The present invention provides a divalent europium activated alkalineearth metal halide phosphor which is different from the above-mentionedknown divalent europium activated alkaline earth metal fluorohalidephosphor, and a process for the preparation of the same. The inventionfurther provides a radiation image recording and reproducing method anda radiation image storage panel employing said phosphor.

Accordingly, a principal object of the present invention is to provide anovel divalent europium activated alkaline earth metal halide phosphorand a process for the preparation of the same.

Another object of the present invention is to provide a radiation imagerecording and reproducing method utilizing said novel stimulablephosphor and a radiation image storage panel employing the same.

As a result of study, the present inventors have found that a phosphorshowing stimulated emission as well as spontaneous emission can beobtained by procedures of mixing at least two compounds (startingmaterials for a host) selected from the alkaline earth metal halidegroup consisting of chlorides, bromides and iodides of Ba, Sr and Cawith an europium compound (starting material for an activator) in anappropriate ratio; and firing the obtained mixture at a temperaturewithin the range of 500°-1300° C. in a weak reducing atmosphere.

The phosphor of the invention is a divalent europium: activated alkalineearth metal halide phosphor having the formula (I):

    M.sup.II X.sub.2 ·aM.sup.II X'.sub.2 :xEu.sup.2+  (I)

in which M^(II) is at least one alkaline earth metal selected from thegroup consisting of Ba, Sr and Ca; each of X and X' is at least onehalogen selected from the group consisting of Cl, Br and I, and X≠X';and a and x are numbers satisfying the conditions of 0.1≦a≦10.0 and0<x≦0.2, respectively.

The process for the preparation of the phosphor having the formula (I)of the invention comprises:

mixing starting materials for the phosphor in a stoichiometric ratiocorresponding to the formula (II):

    M.sup.II X.sub.2 ·aM.sup.II X'.sub.2 :xEu         (II)

in which M^(II), X, X', a and x have the same meanings as defined above;and

firing the obtained mixture at a temperature within the range of500°-1300° C. in a weak reducing atmosphere.

The divalent europium activated alkaline earth metal halide phosphorhaving the formula (I) of the present invention gives a stimulatedemission in the near ultraviolet to blue region when excited with anelectromagnetic wave having a wavelength within the range of 450-1000 nmafter exposure to a radiation such as X-rays, ultraviolet rays, cathoderays, γ-rays, α-rays or β-rays.

The phosphor having the formula (I) also gives a spontaneous emission inthe near ultraviolet to blue region when excited with a radiation suchas X-rays, ultraviolet rays or cathode rays.

On the basis of the stimulated emission characteristics of the novelphosphor as described above, the present inventors have furtheraccomplished the following invention.

That is, the radiation image recording and reproducing method comprisessteps of:

(i) causing the divalent europium activated alkaline earth metal halidephosphor having the above formula (I) to absorb a radiation havingpassed through an object or having radiated from an object;

(i) causing a stimulable phosphor to absorb a radiation having passedthrough an object or having radiated from an object;

(ii) exposing said stimulable phosphor to an electromagnetic wave havinga wavelength within the range of 450-1000 nm to release the radiationenergy stored therein as light emission; and

(iii) detecting the emitted light.

The radiation image storage panel of the invention comprises a supportand at least one phosphor layer provided thereon which comprises abinder and a stimulable phosphor dispersed therein, in which at leastone phosphor layer contains the divalent europium activated alkalineearth metal halide phosphor having the above formula (I).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an X-ray diffraction pattern of a BaCl₂ ·BaBr₂ :0.001Eu²⁺phosphor [(a)], which is an example of the divalent europium activatedalkaline earth metal halide phosphor of the present invention, and X-raydiffraction patterns of BaCl₂, BaBr₂ and the known BaFBr:0.001Eu²⁺phosphor [(b), (c) and (d), respectively].

FIGS. 2 shows a stimulation spectrum of the BaCl₂ ·BaBr₂ :0.001Eu²⁺phosphor.

FIGS. 3 shows stimulated emission spectra of the BaCl₂ ·BaBr₂ :0.001Eu²⁺phosphor, a BaCl₂ ·BaI₂ :0.001Eu²⁺ phosphor and a BaBr₂ ·BaI₂ :0.001Eu²⁺phosphor (Curves 1, 2 and 3, respectively), which are examples of thedivalent europium activated alkaline earth metal halide phosphor of thepresent invention.

FIG. 4 shows spontaneous emission spectra of the BaCl₂ ·BaBr₂ :0.001Eu²⁺phosphor, BaCl₂ ·BaI₂ :0.001Eu²⁺ phosphor and BaBr₂ ·BaI₂ :0.001Eu² +phosphor given upon excitation with ultraviolet rays, (Curves 1, 2 and3, respectively), and excitation spectra thereof (Curves 4, 5 and 6,respectively).

FIG. 5 shows a spontaneous emission spectrum of the knownBaFBr:0.001Eu²⁺ phosphor given upon excitation with ultraviolet rays(Curve 1) and an excitation spectrum thereof (Curve 2).

FIG. 6 shows a relationship between a value and an intensity ofstimulated emission upon excitation at 632.8 nm with respect to theBaCl₂ ·aBaBr₂ :0.001Eu²⁺ phosphor.

FIG. 7 shows relationships between a value and an intensity ofstimulated emission upon excitation at 780 nm (dotted line curve) andbetween a value and an intensity of afterglow after termination ofexcitation with X-rays (solid line curve) with respect to the BaCl₂·aBaBr₂ :0.001Eu²⁺ phosphor.

FIG. 8 is a schematic view showing the radiation image recording andreproducing method according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The divalent europium activated alkaline earth metal halide phosphor ofthe present invention can be prepared, for instance, by a processdescribed below.

As starting materials, the following materials can be employed:

(1) at least two alkaline earth metal halides selected from the groupconsisting of BaCl₂, SrCl₂, CaCl₂, BaBr₂, SrBr₂, CaBr₂, BaI₂, SrI₂ andCaI₂ ; and

(2) at least one compound selected from the group consisting of europiumcompounds such as europium halide, europium oxide, europium nitrate andeuropium sulfate.

The above starting materials (1) are two or more kinds of alkaline earthmetal halides having a halogen different from each other. Further,ammonium halide (NH₄ X", in which X" is any one of Cl, Br and I) may beemployed as a flux.

In the process for the preparation of the phosphor of the invention, theabove-mentioned alkaline earth metal halides (1) and europium compound(2) are, in the first place, mixed in the stoichiometric ratiocorresponding to the formula (II):

    M.sup.II X.sub.2 ·aM.sup.II X'.sub.2 :xEu         (II)

in which M^(II) is at least one alkaline earth metal selected from thegroup consisting of Ba, Sr and Ca; each of X and X' is at least onehalogen selected from the group consisting of Cl, Br and I, and X≠X';and a and x are numbers satisfying the conditions of 0.1≦a≦10.0 and0<x≦0.2, respectively.

From the viewpoint of enhancement in the luminance of stimulatedemission and in the luminance of spontaneous emission, the number a inthe formula (II) which indicates the ratio between M^(II) X₂ and M^(II)X'₂ is preferably within the range of 0.3≦a≦3.3, and more preferably of0.5≦a≦2.0. From the same viewpoint, the number x in the formula (II)which indicates the amount of europium activator is preferably withinthe range of 10⁵ ≦x≦10⁻².

From the viewpoint of improvement in the afterglow characteristics ofspontaneous emission upon excitation with X-rays and of enhancement inthe luminance of spontaneous emission and the luminance of stimulatedemission, it is preferable that X and X' in the formula (II) are Cl andBr, respectively and that the number a in the formula (II) whichindicates the ratio between M^(II) Cl₂ and M^(II) Br₂ is within therange of 0.25≦a≦0.8 or 1.2≦a≦6.0. More preferably, the number a iswithin the range of 0.35≦a≦0.7 or 1.4≦a≦4.0. From the same viewpoint,M^(II) in the formula (II) preferably is Ba.

The mixture of starting materials for the phosphor the present inventionis prepared by any one of the following procedures;

(i) simply mixing the starting materials (1) and (2);

(ii) mixing the starting materials (1), heating the obtained mixture ata temperature of not lower than 100° C. for several hours and thenmixing the heat-treated mixture with the starting material (2); and

(iii) mixing the starting materials (1) in the form of a solution,drying the solution by reduced pressure drying, vacuum drying or spraydrying under heating (preferably, 50°-200° C.), and then mixing theobtained dry product with the starting material (2).

Further, as a modification of the above procedure (ii), there may bementioned a procedure comprising mixing the starting materials (1) and(2) and subjecting the obtained mixture to the heating treatment. Asother modification of the procedure (iii), there may be mentioned aprocedure comprising mixing the starting materials (1) and (2) in theform of a solution and subjecting the solution to the drying.

The mixing is carried out using a conventional mixing apparatus such asa variety of mixers, a V-type blender, a ball mill and a rod mill in anycase of the above-described procedures (i), (ii), (iii).

Then, the resulting mixture of the starting materials is placed in aheat-resistant container such as a quartz boat, an alumina crusible or aquartz crusible, and fired in an electric furnace. The temperature forthe firing suitably ranges from 500° to 1300° C., and preferably rangesfrom 700° to 1000° C. The firing period is determined depending upon theamount of the mixture of starting materials, the firing temperature,etc., and suitably ranges from 0.5 to 8 hours. As the firing atmosphere,there can be employed a weak reducing atmosphere such as a nitrogen gasatmosphere containing a small amount of hydrogen gas or a carbon dioxidegas atmosphere containing carbon monoxide gas. A trivalent europiumcompound is generally employed as the above-mentioned starting material(2) and in the firing stage, the trivalent europium contained in themixture is reduced into divalent europium by the weak reducingatmosphere.

Through the firing procedure, a powdery phosphor of the presentinvention is produced. The powdery phosphor thus obtained may beprocessed in a conventional manner involving a variety of procedures forthe preparation of phosphors such as a washing procedure, a dryingprocedure and a sieving procedure.

The phosphor of the present invention prepared in accordance with theabove-described process is a divalent europium activated alkaline earthmetal halide phosphor having the formula (I):

    M.sup.II X.sub.2 ·aM.sup.II X'.sub.2 :xEu.sup.2+  (I)

in which M^(II) is at least one alkaline earth metal selected from thegroup consisting of Ba, Sr and Ca; each of X and X' is at least onehalogen selected from the group consisting of Cl, Br and I, and X≠X';and a and x are numbers satisfying the conditions of 0.1≦a≦10.0 and0<x≦0.2, respectively.

FIG. 1-(a) shows an X-ray diffraction pattern of a divalent europiumactivated barium chlorobromide phosphor (BaCl₂ ·BaBr₂ :Eu²⁺), which isan example of the phosphor prepared by the process of the presentinvention. FIGS. 1-(b), 1-(c) and i-(d) show X-ray diffraction patternsfor comparison, of barium chloride (BaCl₂), barium bromide (BaBr₂) and aknown divalent europium activated barium fluorobromide (BaFBr:Eu²⁺),respectively. These X-ray diffraction patterns are measured using Cu,Kα₁radiation.

As is clear from FIG. 1, the crystal structure of the BaCl₂ ·BaBr₂ :Eu²⁺phosphor of the present invention is quite different from those of BaCl₂and BaBr₂ which are starting materials for the phosphor. It is alsoclear that the crystal structure of the phosphor of the presentinvention is different from that of the known BaFBr:Eu²⁺ phosphor.

It has been confirmed that such difference of the crystal structureappears in other divalent europium activated alkaline earth metal halidephosphors according to the present invention.

The divalent europium activated alkaline earth metal halide phosphor ofthe present invention gives stimulated emission in the near ultravioletto blue region (peak wavelength of the emission: approx. 405 nm) whenexcited with an electromagnetic wave having a wavelength within theregion of 450-1000 nm such as visible light or infrared rays afterexposure to a radiation such as X-rays, ultraviolet rays and cathoderays.

FIGS. 2 shows an example of a stimulation spectrum of the divalenteuropium activated alkaline earth metal halide phosphor of the presentinvention, that is, a stimulation spectrum of the above-mentioned BaCl₂·BaBr₂ :Eu²⁺ phosphor.

As is clear from FIG. 2, the BaCl₂ ·BaBr₂ :Eu²⁺ phosphor givesstimulated emission upon excitation with an electromagnetic wave in thewavelength region of 450-1000 nm after exposure to a radiation.Particularly, the phosphor exhibits stimulated emission of highintensity upon excitation with an electromagnetic wave in the wavelengthregion of 500-850 nm. In this case, the emitted light can be easilyseparated from the stimulating rays. Based on these facts, thewavelength region of an electromagnetic wave employed as stimulatingrays, namely 450-1000 nm, has been decided in the radiation imagerecording and reproducing method of the present invention.

FIGS. 3 shows examples of stimulated emission spectra of the divalenteuropium activated alkaline earth metal halide phosphors according tothe present invention:

Curve 1: stimulated emission spectrum of BaCl₂ ·BaBr₂ :Eu²⁺ phosphor;

Curve 2: stimulated emission spectrum of BaCl₂ ·BaI₂ :Eu²⁺ phosphor; and

Curve 3: stimulated emission spectrum of BaBr₂ ·BaI₂ :Eu²⁺ phosphor.

As is clear from FIG. 3, the divalent europium activated alkaline earthmetal halide phosphors according to the invention give stimulatedemission in the near ultraviolet to blue region, and each peakwavelength of the emission is approx. 405 nm, although it slightlyvaries depending upon the kinds of phosphors.

The stimulation spectrum and stimulated emission spectra of the divalenteuropium activated alkaline earth metal halide phosphors according tothe present invention are illustrated hereinbefore, for the specificphosphors. However, it has been confirmed that other phosphors accordingto the present invention show almost the same stimulation spectra andstimulated emission spectra as those of the above-mentioned phosphors.Thus, they have the similar stimulated emission characteristics to theabove-mentioned phosphors.

The divalent europium activated alkaline earth metal halide phosphor ofthe present invention also gives spontaneous emission in the nearultraviolet to blue region upon excitation with a radiation such asX-rays, ultraviolet rays and cathode rays.

FIG. 4 shows spontaneous emission spectra of a divalent europiumactivated alkaline earth metal halide phosphors according to the presentinvention given upon excitation with ultraviolet rays, and excitationspectra thereof:

Curve 1: spontaneous emission spectrum of BaCl₂ ·BaBr₂ :Eu²⁺ phosphor;

Curve 2: spontaneous emission spectrum of BaCl₂ ·BaI₂ :Eu²⁺ phosphor;

Curve 3: spontaneous emission spectrum of BaBr₂ ·BaI₂ :Eu²⁺ phosphor;

Curve 4: excitation spectrum of BaCl₂ ·BaBr₂ :Eu²⁺ phosphor;

Curve 5: excitation spectrum of BaCl₂ ·BaI₂ :Eu²⁺ phosphor; and

Curve 6: excitation spectrum of BaBr₂ ·BaI₂ :Eu²⁺ phosphor.

As is clear from FIG. 4, the divalent europium activated alkaline earthmetal halide phosphors according to the present invention givespontaneous emission in the near ultraviolet to blue region uponexcitation with ultraviolet rays, having the peak wavelength at approx.405 nm. The spontaneous emission spectra shown in FIG. 4 are almost thesame as the stimulated emission spectra shown in FIG. 3.

FIG. 5 shows a spontaneous emission spectrum of BaFBr:Eu²⁺ phosphorgiven upon excitation with ultraviolet rays (Curve 1), which is oneexample of the aforementioned known M^(II) FX:Eu²⁺ phosphor, and anexcitation spectrum thereof (Curve 2). From FIGS. 4 and 5, it is clearthat each of the spontaneous emission spectra and excitation spectra ofthe phosphors according to the present invention wholly shifts to thelonger wavelength side as compared with the spontaneous emissionspectrum and excitation spectrum of M^(II) FX:Eu²⁺ phosphor,respectively.

The spontaneous emission spectrum upon excitation with ultraviolet raysand the excitation spectrum of the divalent europium activated alkalineearth metal halide phosphor of the invention are illustrated above withrespect to the three kinds of phosphors. Also has been confirmed thatspontaneous emission spectra and excitation spectra of other phosphorsaccording to the present invention are almost the same as those of theabove-stated three kinds of phosphors. It has been further confirmedthat the spontaneous emission spectrum of the phosphor of the inventiongiven upon excitation with X-rays or cathode rays are almost the same asthose given upon excitation with ultraviolet rays which are shown inFIG. 4.

FIG. 6 graphically shows a relationship between a value and an intensityof stimulated emission [emission intensity upon excitation with He-Nelaser beam (wavelength: 632.8 nm) after exposure to X-rays at 80 KVp]with respect to BaCl₂ ·aBaBr₂ :Eu²⁺ phosphor. As is evident from FIG. 6,the BaCl₂ ·aBaBr₂ :Eu²⁺ phosphor having a value within a range of0.1≦a≦10.0 gives stimulated emission. On the basis of this fact, the avalue range of the divalent europium activated alkaline earth metalhalide phosphor of the invention, namely 0.1≦a≦10.0, has been decided.Among the BaCl₂ ·aBaBr₂ :Eu²⁺ phosphor of the present invention having avalue within the range of 0.1≦a≦10.0, the phosphor having a value withina range of 0.3≦a≦3.3 gives stimulated emission of higher intensity, andparticularly, the phosphor having a value within a range of 0.5≦a≦2.0gives stimulated emission of much higher intensity.

It has been confirmed that the BaCl₂ ·aBaBr₂ :Eu² + phosphor has thesame tendency as shown in FIG. 6 with respect to the relationshipbetween a value and an intensity of spontaneous emission. Further, ithas been confirmed that phosphors according to the present inventionother than the BaCl₂ ·aBaBr₂ :Eu²⁺ phosphor have the same tendencies inthe relationships between a value and the intensity of stimulatedemission and between a value and the intensity of spontaneous emissionas shown in FIG. 6.

Further, a divalent europium activated alkaline earth metalchlorobromide phosphor (M^(II) Cl₂ ·aM^(II) Br₂ :xEu²⁺) which isincluded in the phosphor of the invention has the afterglowcharacteristics as shown in FIG. 7. In this specification, the afterglowis one given after terminating the excitation with a radiation such asX-rays, that is, an afterglow of spontaneous emission.

FIG. 7 shows relationships between a value and an intensity of afterglowwhen excited with X-rays (full line curve) and between a value and anintensity of stimulated emission [emission intensity upon excitationwith semiconductor laser beam (wavelength: 780 nm) after exposure toX-rays at 80 KVp] (dotted line curve) with respect to the BaCl₂ ·aBaBr₂:0.001Eu²⁺ phosphor.

The afterglow which is given by a phosphor after termination of exposureto a radiation such as X-rays to excite it often brings about decreaseof signal-to-noise ratio of an image provided by a radiographicintensifying screen or a radiation image storage panel employing thephosphor. Accordingly, it is desired that the afterglow of spontaneousemission given by the phosphor upon excitation with X-rays is as smallas possible.

When the phosphor is employed in the form of a radiographic intensifyingscreen in the conventional radiography or in the form of a radiationimage storage panel in the radiation image recording and reproducingmethod, the screen or panel can be repeatedly used. For example, theconventional radiography is continuously performed changing only aradiographic film under an intensifying screen fixed to a cassette, sothat the afterglow of the phosphor contained in the screen which isproduced in the previous use causes decrease the S/N ratio of theobtained image.

In the radiation image recording and reproducing method, imageinformation is obtained by exposing the panel to a radiation to storethe radiation energy therein and then irradiating the panel withstimulating rays (e.g., scanning the panel with laser beam) tosequentially read out stimulated emission. When the reading out of thepanel is carried out immediately after exposure to the radiation, theafterglow of the phosphor contained in the panel which is continuouslygiven after terminating the exposure causes decrease the S/N ratio ofthe obtained image.

As is evident from FIG. 7, the BaCl₂ ·aBaBr₂ :Eu²⁺ phosphor having avalue within ranges of 0.25≦a≦0.8 and 1.2≦a≦6.0 shows an afterglow oflower intensity, that is, being improved in the afterglowcharacteristics. Further, the BaCl₂ ·aBaBr₂ :Eu²⁺ phosphor having avalue within ranges of 0.35≦a≦0.7 and 1.4≦a≦4.0 shows the afterglow ofremarkably low intensity as well as shows stimulation emission of highintensity. Particularly, in the phospohr having a value within the rangeof 1.4≦a≦4.0, not only the afterglow after terminating the exposure toX-rays is reduced to remarkably lower level, but also the intensity ofstimulated emission upon excitation with the electromagnetic wave havingthe long wavelength of 780 nm after exposure to X-rays is kept in thehigh level.

Also has been confirmed that the BaCl₂ ·aBaBr₂ :Eu²⁺ phosphor having avalue within the above-mentioned ranges shows the afterglow of reducedintensity without not so decreasing the intensity of the emission, whenthe phosphor is excited with X-rays.

From the viewpoint of emission properties described above, the phosphorof the invention is very useful as a phosphor for the use in a radiationimage storage panel employed in the radiation image recording andreproducing method or for a radiographic intensifying screen employed inthe conventional radiography, both panel and screen being used inmedical radiography such as X-ray photography for medical diagnosis andindustrial radiography for non-destructive inspection.

Particularly in the case of employing the phosphor of the invention inthe radiation image recording and reproducing method, it is possible tovary the wavelength of stimulating rays for exciting the phosphorbecause of the wide wavelength region of its stimulation spectrum,namely 450-1000 nm. It means that a source of stimulating rays can besuitably selected according to the purpose. For example, a semiconductorlaser (having a wavelength in the infrared region) which is in a smallsize and needs only weak driving power can be employed as the source ofstimulating rays, and accordingly the system for performing the methodcan be made compact. From the viewpoint of the intensity of stimulatedemission and of the separation on wavelength between the emitted lightand stimulation rays, the stimulating rays are preferred to be anelectromagnetic wave having a wavelength within the range of 500-850 nm.

The divalent europium activated alkaline earth metal halide phosphorhaving the formula (I), which has the aforementioned crystal structureand emission characteristics, is preferably employed in the form of aradiation image storage panel (also referred as a stimulable phosphorsheet) in the radiation image recording and reproducing method of theinvention. The radiation image storage panel comprises a support and atleast one phosphor layer provided on one surface of the support. Thephosphor layer comprises a binder and a stimulable phosphor dispersedtherein. Further, a transparent protective film is generally provided onthe free surface of the phosphor layer (surface not facing the support)to keep the phosphor layer from chemical deterioration or physicalshock.

In the radiation image recording and reproducing method employing thestimulable phosphor having the formula (I) in the form of a radiationimage storage panel a radiation having passed through an object orradiated from an object is absorbed by the phosphor layer of the panelto form a radiation image as a radiation energy-stored image on thepanel. The panel is then excited (e.g., scanned) with an electromagneticwave in the wavelength region of 450-1000 nm to release the stored imageas stimulated emission. The emitted light is photoelectrically detectedto obtain electric signals so that the radiation image of the object canbe reproduced as a visible image from the obtained electric signals.

The radiation image recording and reproducing method of the presentinvention will be described in more detail with respect to an example ofa radiation image storage panel containing the stimulable phosphorhaving the formula (I), by referring to a schematic view shown in FIG.8.

In FIG. 8 which shows the total system of the radiation image recordingand reproducing method of the invention, a radiation generating device11 such as an X-ray source provides a radiation for irradiating anobject 12 therewith; a radiation image storage panel 13 containing thestimulable phosphor having the formula (I) absorbs and stores theradiation having passed through the object 12; a source of stimulatingrays 14 provides an electromagnetic wave for releasing the radiationenergy stored in the panel 13 as light emission; a photosensor 15 suchas a photomultiplier faces the panel 13 for detecting the light emittedby the panel 13 and converting it to electric signals; an imagereproducing device 16 is connected with the photosensor 15 to reproducea radiation image from the electric signals detected by the photosensor15; a display device 17 is connected with the reproducing device 16 todisplay the reproduced image in the form of a visible image on a CRT orthe like; and a filter 18 is disposed in front of the photosensor 15 tocut off the stimulating rays reflected by the panel 13 and allow onlythe light emitted by the panel 13 to pass through.

FIG. 8 illustrates an example of the system according to the method ofthe invention employed for obtaining a radiation-transmission image ofan object. However, in the case that the object 12 itself emits aradiation, it is unnecessary to install the above-mentioned radiationgenerating device 11. Further, the photosensor 15 to the display device17 in the system can be replaced with other appropriate devices whichcan reproduce a radiation image having the information of the object 12from the light emitted by the panel 13.

Referring to FIG. 8, when the object 12 is exposed to a radiation suchas X-rays provided by the radiation generating device 11, the radiationpasses through the object 12 in proportion to the radiationtransmittance of each portion of the object. The radiation having passedthrough the object 12 impinges upon the radiation image storage panel13, and is absorbed by the phosphor layer of the panel 13. Thus, aradiation energy-stored image (a kind of latent image) corresponding tothe radiation-transmission image of the object 12 is formed on the panel13.

Thereafter, when the radiation image storage panel 13 is irradiated withan electromagnetic wave having the wavelength within the range of450-1000 nm, which is provided by the source of stimulating rays 14, theradiation energy-stored image formed on the panel 13 is released aslight emission. The intensity of so released light is in proportion tothe intensity of the radiation energy which has been absorbed by thephosphor layer of the panel 13. The light signals corresponding to theintensity of the emitted light are converted to electric signals bymeans of the photosensor 15, the electric signals are reproduced as animage in the image reproducing device 16, and the reproduced image isdisplayed on the display device 17.

The operation of reading out the image information stored in theradiation image storage panel is generally carried out by sequentiallyscanning the panel with laser beam and detecting the light emitted underthe scanning with a photosensor such as photomultiplier through a lightguiding means to obtain electric signals. In order to obtain a wellreadable visible image, the read-out operation may comprise apreliminary read-out operation of irradiating the panel with stimulatingrays having energy lower than that in a final read-out operation and thefinal read-out operation of irradiating the panel with stimulating rays(see: Japanese Patent Provisional Publication No. 58(1983)-67240). Theread-out condition in the final read-out operation can be suitably setbased on the result obtained by the preliminary read-out operation.

As the photosensor, solid-state photoelectric conversion devices such asa photoconductor and a photodiode can be also used (see: Japanese PatentApplication No. 58(1983)-86226, No. 58(1983)-86227, No. 58(1983)-219313and No. 58(1983)-219314, and Japanese Patent Provisional Publication No.58(1983)-121874). For example, the photosensor is divided into a greatnumber of pixels, which may be combined with a radiation image storagepanel or positioned in the vicinity of the panel. Otherwise, thephotosensor may be a linesensor in which plural pixels are linearlyconnected or may be such one that corresponds to one pixel.

In the above-mentioned cases, there may be employed for the source ofstimulating rays a linear light source such as an array in which lightemitting diodes (LED), semiconductor lasers or the like are linearlyarranged, in addition to a point light source such as a laser. Theread-out using such photosensor can prevent loss of the light emitted bya panel and can bring about the enhancement of S/N ratio of the image,because the photosensor can receive the emitted light with a largeangle. It is also possible to enhance the read-out speed, becauseelectric signals are sequentially obtained not by scanning the panelwith stimulating rays, but by electrical processing of the photosensor.

After reading out the image information stored in a radiation imagestorage panel, the panel is preferably subjected to a procedure oferasing the radiation energy remaining therein, that is, to the exposureto light having a wavelength in the wavelength region of stimulatingrays for the phosphor contained therein or to heating (see: JapanesePatent Provisional Publication No. 56(1981)-11392 and No.56(1981)-12599). It can prevent the occurring of noise due to the afterimage in the next use of the panel by carrying out the erasingprocedure. Further, the panel can be more effectively prevented from theoccurrence of noise attributable to natural radiations by carrying outthe erasing procedure two times, namely after the read-out and justbefore the next use (see: Japanese Patent Provisional Publication No.57(1982)-116300).

In the radiation image recording and reproducing method of the presentinvention, there is no specific limitation on the radiation employablefor exposure of an object to obtain a radiation transmittance imagethereof, as far as the above-described phosphor gives stimulatedemission upon excitation with the electromagnetic wave after exposure tothe radiation. Examples of the radiation employable in the inventioninclude those generally known, such as X-rays, cathode rays andultraviolet rays. Likewise, there is no specific limitation on theradiation radiating from an object for obtaining a radiation imagethereof, as far as the radiation can be absorbed by the above-describedphosphor to serve as an energy source for producing the stimulatedemission. Examples of the radiation include γ-rays, α-rays and β-rays.

As the source of stimulating rays for exciting the phosphor which hasabsorbed the radiation having passed through or radiated from theobject, there can be employed, for instance, light sources providinglight having a band spectrum distribution in the wavelength region of450-1000 nm; and light sources providing light having a singlewavelength or more in said region such as an Ar ion laser, a Kr ionlaser, a He-Ne laser, a ruby laser, a semiconductor laser, a glasslaser, a YAG laser, a dye laser and a light emitting diode. Among theabove-mentioned sources of stimulating rays, the lasers are preferredbecause the radiation image storage panel is exposed thereto with a highenergy density per unit area. Particularly preferred are a He-Ne laserand an Ar ion laser. The semiconductor laser is also preferred, becauseits size is small, it can be driven by a weak electric power and itsoutput power can be easily stabilized because of the direct modulationthereof.

As the light source for erasing the radiation energy remaining in theradiation image storage panel, a light source at least providing lightof a wavelength within the wavelength region of stimulating rays for theabove-mentioned phosphor is employed. Examples of the light sourceemployable in the method of the present invention include a fluorescentlamp, a tungsten lamp and a halogen lamp as well as the above-mentionedsources of stimulating rays.

The recording and read-out of a radiation image in the method of theinvention can be carried out by using a built-in type radiation imageconversion apparatus which comprises a recording section for recordingthe radiation image on the radiation image storage panel (i.e., causinga stimulable phosphor of the panel to absorb and store radiationenergy), a read-out section for reading out the radiation image recordedon the panel (i.e., irradiating the phosphor with stimulating rays torelease the radiation energy as light emission), and an erasure sectionfor eliminate the radiation image remained in the panel (i.e., causingthe phosphor to release the remaining energy) (see: Japanese PatentApplication No. 57(1982)-84436 and No. 58(1983)-66730). By employingsuch built-in type apparatus, the radiation image storage panel (or arecording medium containing a stimulable phosphor) can be circularly andrepeatedly used and a number of images having a quality at a certainlevel can be stably obtained. The radiation image conversion apparatuscan be made so compact and light weight as to easily set and move theapparatus. It is further possible to move the apparatus place to placeto record the radiation images for mass examinations by loading atraveling X-ray diagnosis station in the form of a vehicle like a buswith the apparatus.

The radiation image storage panel employable in the radiation imagerecording and reproducing method of the invention will be described.

The radiation image storage panel, as described hereinbefore, comprisesa support and at least one phosphor layer provided thereon whichcomprises a binder and the above-described divalent europium activatedalkaline earth metal halide phosphor having the formula (I) dispersedtherein.

The radiation image storage panel having such structure can be prepared,for instance, in the manner described below.

Examples of the binder to be employed in the phosphor layer include:natural polymers such as proteins (e.g. gelatin), polysaccharides (e.g.dextran) and gum arabic; and synthetic polymers such as polyvinylbutyral, polyvinyl acetate, nitrocellulose, ethylcellulose, vinylidenechloride-vinyl chloride copolymer, polyalkyl (meth)acrylate, vinylchloride-vinyl acetate copolymer, polyurethane, cellulose acetatebutyrate, polyvinyl alcohol, and linear polyester. Particularlypreferred are nitrocellulose, linear polyester, polyalkyl(meth)acrylate, a mixture of nitrocellulose and linear polyester, and amixture of nitrocellulose and polyalkyl (meth)acrylate.

The phosphor layer can be formed on a support, for instance, by thefollowing procedure.

In the first place, the stimulable phosphor particles and a binder areadded to an appropriate solvent, and then they are mixed to prepare acoating dispersion of the phosphor particles in the binder solution.

Examples of the solvent employable in the preparation of the coatingdispersion include lower alcohols such as methanol, ethanol, n-propanoland n-butanol; chlorinated hydrocarbons such as methylene chloride andethylene chloride; ketones such as acetone, methyl ethyl ketone andmethyl isobutyl ketone; esters of lower alcohols with lower aliphaticacids such as methyl acetate, ethyl acetate and butyl acetate; etherssuch as dioxane, ethylene glycol monoethylether and ethylene glycolmonoethyl ether; and mixtures of the above-mentioned compounds.

The ratio between the binder and the phosphor in the coating dispersionmay be determined according to the characteristics of the aimedradiation image storage panel and the nature of the phosphor employed.Generally, the ratio therebetween is within the range of from 1:1 to1:100 (binder:phosphor, by weight), preferably from 1:8 to 1:40.

The coating dispersion may contain a dispersing agent to assist thedispersibility of the phosphor particles therein, and also contain avariety of additives such as a plasticizer for increasing the bondingbetween the binder and the phosphor particles in the phosphor layer.Examples of the dispersing agent include phthalic acid, stearic acid,caproic acid and a hydrophobic surface active agent. Examples of theplasticizer include phosphates such as triphenyl phosphate, tricresylphosphate and diphenyl phosphate; phthalates such as diethyl phthalateand dimethoxyethyl phthalate; glycolates such as ethylphthalyl ethylglycolate and butylphthalyl butyl glycolate; and polyesters ofpolyethylene glycols with aliphatic dicarboxylic acids such as polyesterof triethylene glycol with adipic acid and polyester of diethyleneglycol with succinic acid.

The coating dispersion containing the phosphor particles and the binderprepared as described above is applied evenly to the surface of asupport to form a layer of the coating dispersion. The coating procedurecan be carried out by a conventional method such as a method using adoctor blade, a roll coater or a knife coater.

A support material employed in the present invention can be selectedfrom those employed in the conventional radiogaphic intensifying screensor those employed in the known radiation image storage panels. Examplesof the support material include plastic films such as films of celluloseacetate, polyester, polyethylene terephthalate, polyamide, polyimide,triacetate and polycarbonate; metal sheets such as aluminum foil andaluminum alloy foil; ordinary papers; baryta paper; resin-coated papers;pigment papers containing titanium dioxide or the like; and papers sizedwith polyvinyl alcohol or the like. From the viewpoint ofcharacteristics of a radiation image storage panel as an informationrecording material, a plastic film is preferably employed as the supportmaterial of the invention. The plastic film may contain alight-absorbing material such as carbon black, or may contain alight-reflecting material such as titanium dioxide. The former isappropriate for preparing a high-sharpness type radiation image storagepanel, while the latter is appropriate for preparing a high-sensitivetype radiation image storage panel.

In the preparation of a known radiation image storage panel, one or moreadditional layers are occasionally provided between the support and thephosphor layer, so as to enhance the adhesion between the support andthe phosphor layer, or to improve the sensitivity of the panel or thequality of an image provided thereby. For instance, a subbing layer oran adhesive layer may be provided by coating a polymer material such asgelatin over the surface of the support on the phosphor layer side.Otherwise, a light-reflecting layer or a light-absorbing layer may beprovided by forming a polymer material layer containing alight-reflecting material such as titanium dioxide or a light-absorbingmaterial such as carbon black. In the invention, one or more of theseadditional layers may be provided.

As described in U.S. patent application Ser. No. 496,278 (the wholecontent of which is described in European Patent Publication No. 92241),the phosphor layer-side surface of the support (or the surface of anadhesive layer, light-reflecting layer, or light-absorbing layer in thecase that such layers are provided on the phosphor layer) may beprovided with protruded and depressed portions for enhancement of thesharpness of radiation image, and the constitution of those protrudedand depressed portions can be selected depending on the purpose of theradiation image storage panel.

After applying the coating dispersion to the support as described above,the coating dispersion is then heated slowly to dryness so as tocomplete the formation of a phosphor layer. The thickness of thephosphor layer varies depending upon the characteristics of the aimedradiation image storage panel, the nature of the phosphor, the ratiobetween the binder and the phosphor, etc. Generally, the thickness ofthe phosphor layer is within the range of from 20 μm to 1 mm, preferablyfrom 50 to 500 μm.

The phosphor layer can be provided on the support by the methods otherthan that given in the above. For instance, the phosphor layer isinitially prepared on a sheet (false support) such as a glass plate,metal plate or plastic sheet using the aforementioned coating dispersionand then thus prepared phosphor layer is overlaid on the genuine supportby pressing or using an adhesive agent.

The phosphor layer placed on the support can be in the form of a singlelayer or in the form of plural (two or more) layers. When the pluralphosphor layers are placed, at least one layer contains theaforementioned divalent europium activated alkaline earth metal halidephosphor having the formula (I), and the plural layers may be placed insuch a manner that a layer nearer to the surface shows stimulatedemission of higher intensity. In any case, that is, in either the singlephosphor layer or plural phosphor layers, a variety of known stimulablephosphors are employable in combination with the above-mentionedstimulable phosphor.

Examples of the stimulable phosphor employable in combination with theaforementioned stimulable phosphor in the radiation image storage panelof the present invention include the aforementioned M^(II) FX:Eu²⁺phosphor and the phosphors described below;

ZnS:Cu,Pb, BaO·xAl₂ O₃ :Eu, in which x is a number satisfying thecondition of 0.8≦x≦10, and M^(II) O·xSiO₂ :A, in which M^(II) is atleast one divalent metal selected from the group consisting of Mg, Ca,Sr, Zn, Cd and Ba, A is at least one element selected from the groupconsisting of Ce, Tb, Eu, Tm, Pb, Tl, Bi and Mn, and x is a numbersatisfying the condition of 0.5≦x≦2.5, as described in U.S. Pat. No.4,326,078;

(Ba_(1-x-y),Mg_(x),Ca_(y))FX:aEu²⁺, in which X is at least one elementselected from the group consisting of Cl and Br, x and y are numberssatisfying the conditions of 0<x+y≦0.6, and xy≠0, and a is a numbersatisfying the condition of 10⁻⁶ ≦a≦5×10⁻², as described in JapanesePatent Provisional Publication No. 55(1980)-12143; and

LnOX:xA, in which Ln is at least one element selected from the groupconsisting of La, Y, Gd and Lu, X is at least one element selected fromthe group consisting of Cl and Br, A is at least one element selectedfrom the group consisting of Ce and Tb, and x is a number satisfying thecondition of 0<x<0.1, as described in the above-mentioned U.S. Pat. No.4,236,078.

A radiation image storage panel generally has a transparent film on afree surface of a phosphor layer to physically and chemically protectthe phosphor layer. In the panel of the present invention, it ispreferable to provide a transparent film for the same purpose.

The transparent film can be provided on the phosphor layer by coatingthe surface of the phosphor layer with a solution of a transparentpolymer such as a cellulose derivative (e.g. cellulose acetate ornitrocellulose), or a synthetic polymer (e.g. polymethyl methacrylate,polyvinyl butyral, polyvinyl formal, polycarbonate, polyvinyl acetate,or vinyl chloride-vinyl acetate copolymer), and drying the coatedsolution. Alternatively, the transparent film can be provided on thephosphor layer by beforehand preparing it from a polymer such aspolyethylene terephthalate, polyethylene, polyvinylidene chloride orpolyamide, followed by placing and fixing it onto the phosphor layerwith an appropriate adhesive agent. The transparent protective filmpreferably has a thickness within the range of approximately 0.1 to 20μm.

The radiation image storage panel of the present invention may becolored with an appropriate colorant to improve the sharpness of theimage provided thereby, as described in European Patent Publication No.0021174 and Japanese Patent Provisional Publication No. 57(1982)-96300.For the same purpose, white powder may be dispersed in the phosphorlayer of the panel as described in Japanese Patent ProvisionalPublication No. 55(1980)-140447.

The present invention will be illustrated by the following examples, butthese examples by no means restrict the invention.

EXAMPLE 1

To 800 ml of distilled water (H₂ O) were added 333.2 g. of bariumbromide (BaBr₂.2H₂ O), 244.3 g. of barium chloride (BaCl₂.2H₂ O) and0.783 g. of europium bromide (EuBr₃), and they were mixed to obtain anaqueous solution. The aqueous solution was dried at 60° C. under reducedpressure for 3 hours and further dried at 150° C. under vacuum foranother 3 hours to obtain a mixture of the starting materials for thepreparation of a phosphor.

The mixture thus obtained was placed in an alumina crusible, which was,in turn, placed in a high-temperature electric furnace. The mixture wasthen fired at 900° C. for 1.5 hours under a carbon dioxide atmospherecontaining carbon monoxide. After the firing was complete, the crusiblewas taken out of the furnace and allowed to stand for cooling. Thus, apowdery divalent europium activated barium chlorobromide phosphor (BaCl₂·BaBr₂ :0.001Eu²⁺) was obtained.

EXAMPLE 2

The procedure of Example 1 was repeated except for using 427.2 g. ofbarium iodide (BaI₂ ·2H₂ O) instead of barium bromide, to obtain apowdery divalent europium activated barium chloroiodide phosphor (BaCl₂·BaI₂ :0.001Eu²⁺).

EXAMPLE 3

The procedure of Example 1 was repeated except for using 427.2 g. ofbarium iodide (BaI₂ ·2H₂ O) instead of barium chloride, to obtain apowdery divalent europium activated barium bromoiodide phosphor (BaBr₂·BaI₂ :0.001Eu²⁺)

The phosphors prepared in Examples 1 through 3 were excited withultraviolet rays to measure spontaneous emission spectra and excitationspectra.

The results are shown in FIG. 4.

In FIG. 4, Curves 1 to 6 correspond to the following specta:

1: spontaneous emission spectrum of BaCl₂ ·BaBr₂ :0.001Eu²⁺ phosphor(Example 1);

2: spontaneous emission spectrum of BaCl₂ ·BaI₂ :0.001Eu²⁺ phosphor(Example 2);

3: spontaneous emission spectrum of BaBr₂ ·BaI₂ :0.001Eu²⁺ phosphor(Example 3);

4: excitation spectrum of BaCl₂ ·BaBr₂ :0.001Eu²⁺ phosphor (Example 1);

5:excitation spectrum of BaCl₂ ·BaI₂ :0.001Eu²⁺ phosphor (Example 2);and

6: excitation spectrum of BaBr₂ ·BaI₂ :0.001Eu²⁺ phosphor (Example 3).

The phosphors prepared in Examples 1 through 3 were excited to X-rays toevaluate the intensity of spontaneous emission.

The results on the evaluation of the phosphors are set forth in Table 1,in which the result on the known BaFBr:0.001Eu²⁺ phosphor given upon thesame excitation is also set forth for comparison.

                  TABLE 1                                                         ______________________________________                                                        Relative Intensity of                                                         Spontaneous Emission                                          ______________________________________                                        Example                                                                        1                100                                                          2                260                                                          3                290                                                         BaFBr: 0.001Eu.sup.2+ phosphor                                                                  100                                                         ______________________________________                                    

Further, the phosphors prepared in Examples 1 through 3 were excitedwith an He-Ne laser (oscillation wavelength: 632. 8 nm) after exposureto X-rays at 80 KVp, to measure stimulated emission spectra. The resultsare shown in FIG. 3.

In FIG. 3, Curves 1 to 3 correspond to the following spectra:

1: stimulated emission spectrum of BaCl₂ ·BaBr₂ :0.001Eu²⁺ phosphor(Example 1);

2: stimulated emission spectrum of BaCl₂ ·BaI₂ :0.001Eu²⁺ phosphor(Example 2);

3: stimulated emission spectrum of BaBr₂ ·BaI₂ :0.001Eu²⁺ phosphor(Example 3);

The phosphor prepared in Example 1 was excited with light whosewavelength was varied in the range of 450-1100 nm after exposure toX-rays at 80 KVp, to measure stimulation spectrum at the emissionwavelength of 405 nm. The results are shown in FIG. 2.

Furthermore, the phosphors prepared in Examples 1 through 3 were excitedwith light of 780 nm after exposure to X-rays at 80 KVp, to evaluate theintensity of stimulated emission.

The results on the evaluation of the phosphors are set forth in Table 2,in which the result on the known BaFBr:0.001Eu²⁺ phosphor given underthe same conditions is also set forth for comparison.

                  TABLE 2                                                         ______________________________________                                                        Relative Intensity of                                                         Stimulated Emission                                           ______________________________________                                        Example                                                                        1                700                                                          2                 70                                                          3                 70                                                         BaFBr: 0.001Eu.sup.2+  phosphor                                                                 100                                                         ______________________________________                                    

EXAMPLES 4-9

The procedure of Example 1 was repeated except for using barium chlorideand barium bromide at the amounts set forth in Table 3, respectively, toobtain a variety of powdery divalent europium activated bariumchlorobromide phosphor (BaCl₂ ·aBaBr₂ :0.001Eu²⁺).

                  TABLE 3                                                         ______________________________________                                               Barium   Barium                                                        Example                                                                              Bromide  Chloride Obtained Phosphor                                    ______________________________________                                        4       66.6 g  439.7 g  BaCl.sub.2 · 0.11BaBr.sub.2 :                                        0.001Eu.sup.2+                                       5      166.6 g  366.5 g  BaCl.sub.2 · 0.33BaBr.sub.2 :                                        0.001Eu.sup.2+                                       6      433.2 g  171.0 g  BaCl.sub.2 · 1.9BaBr.sub.2 :                                         0.001Eu.sup.2+                                       7      499.8 g  122.2 g  BaCl.sub.2 · 3.0BaBr.sub.2 :                                         0.001Eu.sup.2+                                       8      533.1 g   97.7 g  BaCl.sub.2 · 4.0BaBr.sub.2 :                                         0.001Eu.sup.2+                                       9      599.8 g   48.9 g  BaCl.sub.2 · 9.0BaBr.sub.2 :                                         0.001Eu.sup.2+                                       ______________________________________                                    

The phosphors prepared in Example 1 and Examples 4 through 9 weremeasured on the intensity of afterglow at 10 sec. after exposure toX-rays at 80 KVp. The phosphors were also measured on the intensity ofstimulated emission when excited with a semiconductor laser (780 nm)after exposure to X-rays at 80 KVp.

The results are graphically shown in FIG. 7.

In FIG. 7, the solid line curve shows a relationship between a value andan intensity of afterglow after termination of excitation with X-raysand the dotted line curve shows a relationship between a value and anintensity of stimulated emission with respect to the BaCl₂ ·aBaBr₂:0.001Eu²⁺ phosphor.

As is evident from FIG. 7, the BaCl₂ ·aBaBr₂ :Eu²⁺ phosphor having avalue within ranges of 0.25≦a≦0.8 and 1.2≦a≦6.0 showed an afterglow ofreduced intensity. Further, the BaCl₂ ·aBaBr₂ :Eu²⁺ phosphor having avalue within ranges of 0.35≦a≦0.7 and 1.4≦a≦4.0 showed the afterglow ofnoticeably low intensity without not so decreasing the intensity ofstimulated emission, as compared with the BaCl₂ ·BaBr₂ :Eu²⁺ phosphor(a=1, Example 1).

Particularly, the phosphor having a value within the range of 1.4≦a≦4.0gave stimulated emission of high intensity upon excitation at thewavelength of 780 nm and was reduced on the afterglow to a noticeablylow level.

EXAMPLE 10

To a mixture of the powdery divalent europium activated bariumchlorobromide phosphor (BaCl₂ ·BaBr₂ :0.001Eu²⁺) obtained in Example 1and a linear polyester resin were added successively methyl ethyl ketoneand nitrocellulose (nitrification degree: 11.5%), to prepare adispersion containing the phosphor and the binder (10:1, by weight).Subsequently, tricresyl phosphate, N-butanol and methyl ethyl ketonewere added to the dispersion. The mixture was sufficiently stirred bymeans of a propeller agitater to obtain a homogeneous coating dispersionhaving a viscosity of 25-35 PS (at 25° C.).

The coating dispersion was applied to a polyethylene terephthalate sheetcontaining titanium dioxide (support, thickness: 250 μm) placedhorizontally on a glass plate. The application of the coating dispersionwas carried out using a doctor blade. The support having a layer of thecoating dispersion was then placed in an oven and heated at atemperature gradually rising from 25° to 100° C. Thus, a phosphor layerhaving a thickness of 250 μm was formed on the support.

On the phosphor layer was placed a transparent polyethyleneterephthalate film (thickness: 12 μm; provided with a polyester adhesivelayer on one surface) to combine the transparent film and the phosphorlayer with the adhesive layer.

Thus, a radiation image storage panel consisting essentially of asupport, a phosphor layer and a transparent protective film wasprepared.

The radiation image storage panel prepared in Example 10 was measured onthe sensitivity (i.e., intensity of stimulated emission) when excitedwith light of 780 nm after exposure to X-rays at 80 KVp.

The result on the evaluation of the panel is set forth in Table 4, inwhich the result on a radiation image storage panel prepared in the samemanner as Example 10 except for employing the known BaFBr:0.001Eu²⁺phosphor, being given under the same conditions, is also set forth forcomparison.

                  TABLE 4                                                         ______________________________________                                                           Relative Sensitivity                                       ______________________________________                                        Example 10           700                                                      Radiation image storage panel                                                                      100                                                      employing BaFBr: 0.001Eu.sup.2+ phosphor                                      ______________________________________                                    

EXAMPLE 11

The procedure of Example 10 was repeated except for employing the BaCl₂·1.9BaBr₂ :0.001Eu²⁺ phosphor obtained in Example 6 instead of BaCl₂·BaBr₂ :0.001Eu²⁺ phosphor, to obtain a radiation image storage panelconsisting essentially of a support, a phosphor layer and a transparentprotective film.

The radiation image storage panels prepared in Examples 10 and 11 weremeasured on the intensity of afterglow at 10 sec. after terminating theexposure to X-rays at 80 KVp. The panels were also measured on thesensitivity (i.e., intensity of stimulated emission) when excited with asemiconductor laser beam (780 nm) after exposure to X-rays at 80 KVp.

The results on the evaluation of the panels are set forth in Table 5.

                  TABLE 5                                                         ______________________________________                                                   Relative Intensity                                                                       Relative                                                           of Afterglow                                                                             Sensitivity                                             ______________________________________                                        Example                                                                       10           1.0          1.0                                                 11           0.23         0.8                                                 ______________________________________                                    

We claim:
 1. A radiation image recording and reproducing methodcomprising steps of:(i) causing a divalent europium activated alkalineearth metal halide stimulable phosphor having the formula (I):

    M.sup.II X.sub.2 ·aM.sup.II X'.sub.2 :xEu.sup.2+  (I)

in which M^(II) is at least one alkaline earth metal selected from thegroup consisting of Ba, Sr and Ca; each of X and X' is at least onehalogen selected from the group consisting of Cl, Br and I, and X≠X';and a and x are numbers satisfying the conditions of 0.1≦a≦10.0 and0<x≦0.2, respectively, to absorb a radiation having passed through anobject or having radiated from an object; (ii) exposing said stimluablephosphor to an electromagnetic wave having a wavelength within the rangeof 450-1000 nm to release radiation energy stored therein as lightemission; and (iii) detecting the emitted light.
 2. The radiation imagerecording and reproducing method as claimed in claim 1, in which a inthe formula (I) is a number satisfying the condition of 0.3≦a≦3.3. 3.The radiation image recording and reproducing method as claimed in claim2, in which a in the formula (I) is a number satisfying the condition of0.5≦a≦2.0.
 4. The radiation image recording and reproducing method asclaimed in claim 1, in which each of X and X' in the formula (i) is Clor Br.
 5. The radiation image recording and reproducing method asclaimed in claim 1, in which X and X' in the formula (I) and Cl and Br,respectively, and a in the formula (I) is a number satisfying thecondition of 0.25≦a≦0.8 or 1.2≦a≦6.0.
 6. The radiation image recordingand reproducing method as claimed in claim 5, in which a in the formula(I) is a number satisfying the condition of 0.35≦a≦0.7 or 1.4≦a≦4.0. 7.The radiation image recording and reproducing method as claimed in claim1, in which M^(II) in the formula (I) is Ba.
 8. The radiation imagerecording and reproducing method as claimed in claim 1, in which x inthe formula (I) is a number satisfying the condition of 10⁻⁵ ≦x≦10⁻². 9.The radiation image recording and reproducing method as claimed in claim1, in which said electromagnetic wave is one having a wavelength withinthe range of 500-850 nm.
 10. The radiation image recording andreproducing method as claimed in claim 1, in which said electromagneticwave is a laser beam.